head process

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head pro·cess

the primordium of the notochord.
See also: notochordal process.

head process

a strand of cells that extends forward from the primitive node in the early stages of embryonic development in vertebrates. It is the precursor of the notochord and forms the primitive axis around which the embryo develops. Also called notochordal plate.

head process

An axial strand of cells in vertebrate embryos extending forward from the primitive knot. It forms a primitive axis about which the embryo differentiates.
See also: process


1. the anterior or superior part of a structure or organism, in vertebrates containing the brain and the organs of special sense. See also skull, caput.
2. one animal; used in reference to farm livestock, e.g. ten head of cattle.

articular head
an eminence on a bone by which it articulates with another bone.
head bail
head bob
the patient is unable to keep the head still while at rest, it makes an involuntary, usually at regular intervals, short, quick downward movement then recovers.
head bot
head cap
of spermatozoon, formed by the collapse of the acrosomial vesicle over the nucleus.
head carriage
includes at attention, drooping, rotated, deviated.
head catarrh
malignant catarrhal fever.
head clamp
head collar
a leather halter-like piece of harness consisting of a poll strap, nose band and usually a rope shank.
head cover
hood or cap used by surgical personnel to reduce contamination of the surgical area by hair and associated flora.
head deviation
the poll-nose axis is turned laterally so that the animal walks in circles but there is no disturbance of balance. Occurs in lesions of the neck and of the cerebral cortex.
fetal head lateral deviation
the fetal head is deviated laterally to lie against the shoulder of the fetus, the front feet and the lateral aspect of the neck being presented to the pelvic inlet; a dystocia which can only be relieved by repulsion of the fetus and the return of the head to between the front feet.
fetal head ventral deviation
the head of the fetus is flexed ventrally so that the fore feet and the dorsum of the neck are presented to the pelvic inlet; a dystocia which can only be relieved by repulsion of the fetus and lifting of the head into the pelvic canal.
head grit
jaundice, photosensitization and hepatic injury in lambs caused by Narthecium ossifragum poisoning.
head injury
traumatic injury to the head resulting from a fall or violent blow. Such an injury may be open or closed and may involve a brain concussion, skull fracture, or contusions of the brain.
head mange
nerve head
the optic disk.
head nod
see head bob (above).
head picking
a form of cannibalism in which birds pick at and injure each other's wattles, combs, eyes; beak trimming reduces the problem but does not prevent the vice.
head posture
see head carriage (above).
head pressing
persistent pushing with the head against a fixed object. Part of the dummy syndrome as in hepatic encephalopathy or encephalitis.
head process
the elongating cephalic tissues which represent the first step in the development of the fetal body.
head rotation
twisting of the head around the axis from the poll to the nose. To be differentiated from deviation of the head. Caused usually by lesions of the vestibular apparatus on one side. The animal walks in circles and has problems maintaining its balance.
head shaking
common in dogs and cats with acute inflammation or foreign bodies in the external ear canal. May be a cause of auricular hematoma. Seasonal head shaking is seen in horses mainly during spring, worsening in summer. A trigeminal neuritis caused by increased levels of melatonin is the suspected cause in some. Allergic rhinitis may also be an underlying cause.
head shy
said of a horse that tries to avoid having its head handled or its headstall put on.
head stall
see head collar (above).
head tilt
includes rotation and deviation.
Enlarge picture
Head tilt. By permission from Nelson RW, Couto CG, Small Animal Internal Medicine, Mosby, 2003
head tremor
a feature of cerebellar lesions.
head twist injury
injury to cervical vertebrae may occur in horses as a result of a fall with the head and neck under the body.
head's zones
in acupuncture the zones of human skin which are responsive to abnormalities in each of the vital organs.